Wind sensors have a wide range of practical applications. For example, wind sensors are common at weather stations, airports, and other locations for sensing wind speed and direction. At an airport, as an example, this information is particularly useful in guiding aircraft with respect to take-offs and landings.
An anemometer is one device that is used for measuring wind speed. There are many types of anemometers, such as cup, windmill, and hot-wire anemometers.
In a hot-wire anemometer, a very thin element (having a cross-section on the order of several micrometers) is heated up to a temperature above the ambient. Air flowing past the element has a cooling effect on the element. Since the electrical resistance of most metals is dependent upon the temperature of the metal (tungsten is a popular choice for elements), a mathematical relationship can be obtained between the resistance of the element and the flow velocity. This relationship is well known.
Several ways of implementing a hot-wire anemometer exist, and hot-wire anemometers can be further classified as CCA (Constant-Current Anemometer), CVA (Constant-Voltage Anemometer) and CTA (Constant-Temperature Anemometer). The voltage output from these anemometers is, thus, the result of a circuit within the device trying to maintain constant the specific variable (current, voltage or temperature).
The advantages of thermal anemometry include high accuracy, an easily-automated collection procedure, and a high frequency response allowing measurement of turbulent flows. The use of electrically self-heated resistors, hot wires, and hot films as thermal anemometer transducers is well known in the prior art. These elements, when maintained at constant temperature rise above the ambient, can indicate with precision the direction and magnitude of airflow. This is accomplished electrically by using a servo loop, which senses the error between the voltage divider involving the sensing element and the voltage divider involving a thermistor with a similar temperature coefficient of resistance.
By compensating for ambient temperature changes, the elements are maintained at constant temperature rise above the ambient. Any airflow change across the sensing element requires a change in current through the sensor element to keep it at a constant temperature rise. Platinum wire or depositions of Platinum have been commonly used because of its stability at high temperatures; Platinum has a high temperature coefficient. Since a high temperature coefficient of resistance is required, the material supporting the Platinum needs to have a good high temperature coefficient of resistance as well. Combining the cost of Platinum, the support material, and the sophisticated processes in manufacturing these elements, the costs for producing, using, and maintaining thermal anemometers has been prohibitively high. Combining these considerations with the data processing requirements of such anemometers has made use in the commercial, industrial, and military marketplaces largely impractical.